This invention relates to guided wave radar intrusion detection systems and in particular to an integral enable-disable switching means that permits ingress and egress through selected portions of such a system without affecting overall system integrity and without requiring auxilliary intrusion detection devices.
Over the past decade, there has been a growing effort expended on the development of intrusion detection systems for outdoor use and a variety of systems have been developed for this purpose. A number of sensors, already developed or now under evaluation, monitor changes in either acoustic, optical, infrared, electromagnetic or seismic information when an intruder enters a particular zone of detection and declare the presence of an intruder when one or more of these changes exceeds a certain threshold. Unfortunately, it is not often possible to provide reliable detection under all weather conditions, mainly because of the inability of most systems to accommodate the effects of the environment in the vicinity of the sensor; that is, the detectable characteristics of the intruder too closely resemble the characteristics of either environmental effects or normal man-made interference. These disadvantages have been largely overcome by recently developed guided wave radar intrusion detection systems. A system of this type called Guided Intrusion Detection And Ranging (GUIDAR) is marketed commercially by Computing Device Company, a division of Control Data Canada Ltd.
The GUIDAR concept is based on the properties of so-called leaky (or ported) coaxial cables, in which a portion of the electromagnetic (EM) energy travelling within the cable is allowed to escape in a controlled fashion, creating a travelling EM wave external to but within the vicinity of the cable. Two such cables, spaced a few feet apart, are laid around the perimeter of a securred area. A pulse of EM energy is transmitted down one of the cables, a portion of the energy "leaks out" and irradiates the detection zone. An intruder crossing the zone disturbs the field, causing a reflected wave to couple into the second (receive) cable. The time between the onset of the transmit pulse and the return of the reflected signal provides a measure of the intruder's position.
Although systems of this type represent an improvement over other state-of-the-art intrusion detection systems, they have been found to have problems when the protected zone must be interrupted by one or more entry portals. Currently, it is necessary to permanently disable that area of the system in order to prevent large targets like vehicles from overloading the system processor and producing false alarms in widely separated locations. The approach to the problem of course requires an independent intrusion sensor which is switched on when the entry portal is not in use.
There currently exists therefore, the need for means whereby intruder detection at the portals of guided wave radar intrusion detector systems can be selectively enabled and disabled without permanently disabling the portal section and without requiring an auxiliary detection system. the present invention is directed toward satisfying that need.